The present invention relates to tower coolers designed to cool, by direct contact with the atmospheric air, a liquid, particularly water, falling under the effects of gravity through an exchanger through which the flow of atmospheric air passes approximately horizontally. In this type of cooler, the exchange between the air and the liquid therefore involves cross-flows more particularly the invention relates to an anti-freezing device for this type of cooler.
Examples of the foregoing type of cooler are those disclosed in FR 2 593 902 (EDF--cross-flow cooler with natural draught), U.S. Pat. No. 3,389,895 (De Flon--cross-flow cooler with mechanical draught and splash slats), U.S. Pat. No. 4,020,130 (Ovard--similar to the latter), U.S. Pat. No. 4,320,073 (Marley--exchanger for a cross-flow, film-type cooler with mechanical draught). The water can actually trickle through the exchanger in a film (film trickling), in droplets ("splash" trickling) as the result of the water striking the elements of the exchanger, or else in any other intermediate or combined form of film trickling and droplet trickling.
When the ambient temperature is below 0.degree. C. the water is in danger of freezing, and this must be avoided at all costs inside the cooler, particularly anywhere in the exchanger.
This is because the formation of blocks of ice can cause elements of the exchanger, or even of the structure supporting it, to fracture. On the other hand, when the ambient temperature rises again, the thawing of blocks of ice, beginning in the upper part, will cause these to fall and this could produce substantial damage.
As a result, the designers and operators of tower coolers have constantly sought to delay, reduce and control ice formation.
In this connection the following patents taken out by the Applications themselves may be cited: FR 2 266 134, EP-B 54 843, EP-B 58 109, EP-B 264 316 and FR 2 658 906, as well as EDF's patent FR 2 593 902.
One means frequently employed involves reducing the thermal exchange efficiency by increasing the flow of water locally, whether at the periphery (FR 2 266 134 and FR 2 658 906), at the centre (EP 54 843), or over one sector (FR 2 593 902), and by reducing or even interrupting it elsewhere.
This reduction of thermal efficiency may also be obtained by restricting the airflow, either by artificial means (EP 264 316) or even by ice formation (EP 58 109), suitably controlled (outside the exchanger in the air intake ports). Lastly, efficiency reduction may be obtained by distributing at least some of the hot water that is to be cooled at an intermediate level of the exchanger.
The precautions against freezing must already have been taken before the means temperature of the cooled water approaches zero degree centigrade, because the temperature of the water is not constant throughout the depth of the exchanger, in other words between the external edge of the cooler and the centre of the cooler, essentially owing to the cross-flow exchange.
In the counter-flow coolers this lack of uniformity is insignificant because the counter-flow exchange of the exchanger does not generate differences in cooling, the lack of uniformity merely coming from the water droplets exiting beneath the exchanger, which are cooled by the air entering the cooler before proceeding gradually through the latter by counter-flow.
Conversely, cross-flow coolers are essentially characterised by two-dimensional temperature distribution: in the height direction and in the depth direction of the exchanger.
The temperature profile of the water up the height of the exchanger varies from the front surface of the exchanger where the water is everywhere cooled directly by the atmospheric air, to the rear surface where the water is cooled by reheated air in a non-uniform manner depending on the height of the exchanger; this air is in fact heated to a greater degree the higher up it is in the exchanger, that is to say the closer it is to the hot water supply.
The further away one gets from the hot water distribution platform, that is to say the further the water falls, the greater becomes the temperature difference--being zero at the top of the exchanger--between the water at the front surface and at the rear surface, and in the case of modern industrial coolers said difference is greatest either at the foot of the exchanger or slightly thereabove. The difference in temperature from front to back decreases after attaining a maximum value, because water cooling is limited to the temperature of the air (as shown on a wet bulb thermometer).
For example, for a given industrial cooler, involving water splashing onto gratings across spacings, a 15 K cooling difference (the difference between the temperature of the hot water and the mean temperature of the cold water) and a 10 K cooling approximation (the difference between the mean temperature of the cold water and that of the atmospheric air at the wet bulb), the difference in the water temperature on the same horizontal plane between the rear surface of the exchanger and the front surface passes through a maximum of around 16 K and ends at the water collecting basin with a still very high value of around 14 K, corresponding to an 8 K temperature difference between the mean temperature of the cold water and that of the coldest water.
The difference in water temperature between the front surface and the rear surface may be greater than the cooling difference within the cooler!
In another example of an industrial cooler constituted by four tiers of exchanger/film "packings", the temperature difference of the water between the rear and front surfaces is as follows:
______________________________________ above the first, uppermost tier zero (hot water supply) between the first and second tiers 6.6K between the second and third tiers 10.3K between the third and fourth tiers 12.4K at the foot of the fourth tier (basin) 13.3K ______________________________________
But the temperature of the water on the same horizontal plane (corresponding to the airflow) does not vary linearly from the front surface to the rear surface. The difference between the mean temperature of the water and the lowest water temperature (at the front surface) is generally more than half the difference in the temperatures of the water between the rear and the front of the exchanger. In the last example cited, the difference between the mean temperature underneath the exchanger and the lowest temperature is 8.3 (&gt;13.3/2).
It will therefore be seen that there is already a danger with cross-flow coolers of local ice formation whilst the mean temperature of the cold water is nearly 10.degree. C.
It is therefore the object of the invention to reduce variation in the temperature of the water at one or more intermediate levels of the waterfall through the exchanger, in order to reduce the difference between the mean temperature of the water and the temperature of the coldest water, which is at the front of the exchanger.
Once, as the water is falling, a level is reached at which the lowest temperature of the water (at the front surface) would be less than the mean temperature of the cold water in the cooler, it is clearly desirable to make uniform the temperature of the water at that level, so as to allow the water to be cooled to a mean temperature closer to zero degree without risk of freezing in the front portion at the foot of the cooler.